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We identify, by dislocation theory and molecular dynamics simulations, possible dislocation dipoles (5͉7͉7͉5 and 4͉8͉8͉4) as defect nuclei under tension in boron nitride nanotubes. The formation energies of the dipoles are then evaluated by ab initio gradient-corrected density functional theory. The 5͉7͉7͉5 dipole appears to be more favorable in spite of its homoelemental B-B and N-N bonds. Compared to carbon nanotubes, the formation energy of the primary defect is higher and remains positive at larger strain in boron nitride nanotubes, thus suggesting greater yield resistance.

Density functional theory and ab initio quantum mechanical techniques were employed to study the ring opening of singlet cyclopropylidene 1-S and the internal rotation of singlet allene 2-S. The B3LYP, CISD, and CASSCF(4,4) geometry optimizations used a TZP basis set. Single-point TZ2P energies were evaluated at CCSD(T) and multireference configuration interaction with single and double excitations (MR-CISD) levels. Employing the four most important configurations at the MR-CISD/TZ2P//UB3LYP/TZP level gave singlettriplet separations for 1 of 11.1 kcal mol -1 and for 2 48.9 kcal mol -1 . Asymmetric transition structures for the ring opening of 1-S were obtained at B3LYP/TZP and CISD/TZP. The barriers are 4.2-4.5 kcal mol -1 at CCSD(T)/TZ2P with these geometries. In contrast, only one C s symmetric TS was located at CASSCF-(4,4), and the ring opening barrier is only 3.6 and 3.1 kcal mol -1 at CCSD(T)/TZ2P and MR-CISD/TZ2P, respectively. At CCSD(T)/6-31G* an asymmetric structure for the TS results confirming the predictions of the B3LYP and CISD methods. The B3LYP/6-31G* intrinsic reaction coordinate (IRC) delineated the complex modes of rotation of the methylene groups during the ring opening. Although the overall rotation in going from 1-S to 2-S must be conrotatory, the ring opening of 1-S starts with a disrotatory motion of both methylene groups. However, before the C 1 TS is reached, the sense of rotation of one of the methylene groups changes. At the B3LYP and CASSCF(4,4) levels, the transition state for the internal rotation of allene is a planar bent (C 2V ) open-shell singlet ( 1 A 2 ). The D 2h open-shell singlet is a higher lying second-order stationary point. All D 2h and C 2V closed-shell species are significantly higher in energy. MR-CISD single-point activation barriers obtained from the B3LYP and CASSCF(4,4) geometries were 44.6 and 45.5 kcal mol -1 , respectively. The B3LYP/TZP ring opening (4.8 kcal mol -1 ) and allene internal rotation (44.6 kcal mol -1 ) barriers are in remarkably good agreement with the CCSD(T)/TZ2P//B3LYP/TZP value (4.5 kcal mol -1 ) on the barrier and with the MR-CISD/TZ2P//B3LYP/TZP barrier for the latter (44.6 kcal mol -1 ).

The chemical reaction dynamics to form cyanobenzene C6H5CN(X 1A1), and perdeutero cyanobenzene C6D5CN(X 1A1) via the neutral–neutral reaction of the cyano radical CN(X 2Σ+), with benzene C6H6(X 1A1g) and perdeutero benzene C6D6(X 1A1g), were investigated in crossed molecular beam experiments at collision energies between 19.5 and 34.4 kJ mol−1. The laboratory angular distributions and time-of-flight spectra of the products were recorded at mass to charge ratios m/e=103–98 and 108–98, respectively. Forward-convolution fitting of our experimental data together with electronic structure calculations (B3LYP/6−311+G**) indicate that the reaction is without entrance barrier and governed by an initial attack of the CN radical on the carbon side to the aromatic π electron density of the benzene molecule to form a Cs symmetric C6H6CN(C6D6CN) complex. At all collision energies, the center-of-mass angular distributions are forward–backward symmetric and peak at π/2. This shape documents that the decomposing intermediate has a lifetime longer than its rotational period. The H/D atom is emitted almost perpendicular to the C6H5CN plane, giving preferentially sideways scattering. This experimental finding can be rationalized in light of the electronic structure calculations depicting a H–C–C angle of 101.2° in the exit transition state. The latter is found to be tight and located about 32.8 kJ mol−1 above the products. Our experimentally determined reaction exothermicity of 80–95 kJ mol−1 is in good agreement with the theoretically calculated one of 94.6 kJ mol−1. Neither the C6H6CN adduct nor the stable iso cyanobenzene isomer C6H5NC were found to contribute to the scattering signal. The experimental identification of cyanobenzene gives a strong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn’s moon Titan. This reaction might further present a barrierless route to the formation of heteropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrestrial environments as well as hydrocarbon rich flames.

The thermochemistry of dissociation and elimination reactions of organogallium precursors for the GaN chemical vapor deposition (CVD) is studied at the hybrid Hartree-Fock/density functional level of theory (B3LYP/pVDZ). Geometries, relative energies, vibrational frequencies of R x GaNR′ x species, and their dissociation products (NR x , GaR x , x ) 1-3; (R, R′ ) H, CH 3 ) are presented. Methane elimination from the source adducts is exothermic at standard conditions, while hydrogen elimination is endothermic. Both for R ) H, CH 3 elimination reactions are predicted to be more favorable compared to dissociation into components, in contrast to the halogen containing precursors. The Ga-N bond dissociation enthalpies (kJ mol -1 ) are the highest for R 2 GaNR′ 2 compounds (313-382), followed by ; and for donor-acceptor complexes R 3 GaNR′ 3 (56-100) they are the lowest. (CH 3 ) x GaNH x isomers are more than 50 kJ mol -1 lower in energy than H x GaN(CH 3 ) x species, but the formation of Ga-H and N-H bonds is the thermodynamically most favorable process. Hence, the replacement of alkyl groups might be viable during the CVD process from trimethylgallium and ammonia.

Heptacene was generated by surface-assisted didecarbonylation of an α-diketone precursor on a Ag(111) surface. Monitoring of the surface reaction and characterization of the adsorbed heptacene was performed with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The surface-assisted formation of heptacene occurs around 460 K. Both the heptacene and the precursor molecules are oriented along the high-symmetry directions of the (111) surface and their molecular π systems face towards the substrate. The interaction with the Ag(111) substrate is not laterally uniform, but appears to be strongest on the central part of the molecule, in line with the expectations from Clar's rule. In the STM images, heptacene shows a dumbbell shape, which may correspond to the substantial out-of-plane deformations of heptacene on Ag(111). As revealed by DFT, the center of the molecule is closer to the surface than the outer parts. In addition, the inner rings are most affected by charge redistribution between surface and molecule. Heptacene acts as an acceptor and receives a negative charge of -0.6e from the Ag(111) surface. Since vacuum-sublimable α-diketone precursors for even larger acenes are available, the approach is promising for the on-surface synthesis of higher acene homologues such as octacene and nonacene.

The thermochemistry of association reactions of organogallium precursors for the GaN chemical vapor deposition (CVD) is studied. Geometries, relative energies, and vibrational frequencies of ring and cluster compounds [RGaNR′] n , [R 2 GaNR′ 2 ] m , (n ) 2-4, 6; m ) 2-3; R, R′ ) H, CH 3 ) are obtained at the hybrid Hartree-Fock/density functional level of theory (B3LYP/pVDZ). Formation of the [RGaNR′] 4 tetramer and [RGaNR′] 6 hexamer species is thermodynamically favorable in the gas phase at temperatures up to 720 K (R ) H, R′ ) CH 3 ) and 920-940 K (R ) H, CH 3 , R′ ) H). The thermodynamic analysis of the major gas-phase reactions indicates that association processes might play a key role in the GaN CVD under low-temperaturehigh-pressure conditions.